DC-DC converters are commonly used in integrated circuits for providing stable voltages. A DC-DC converter is an electronic circuit that converts a source of direct current from one voltage level to another. For accurate conversion, the output voltage (converted voltage) of the DC-DC converter needs to be sensed, for example, using a current sensing circuit. The resulting current (sensed current) generated by the current sensing circuit is then converted to a voltage, which is further compared with a reference voltage to determine whether the converted voltage is accurate or not, so that the converted voltage can be adjusted. Accordingly, a feedback loop is formed.
The feedback loop used for sensing and adjusting the converted voltage suffers from stability problem. To solve this problem, the voltage generated by the current sensing circuit was conventionally summed with a saw-tooth voltage before being compared with the reference voltage. FIG. 1 illustrates a circuit diagram of a conventional saw-tooth generator and summation circuit. The saw-tooth generator includes operational amplifier OP2′, transistor M3′, capacitor C′, and current source I_Bias′, which provides bias current I′. When voltage VA′ at node A′ is lower than voltage V_reference, operational amplifier OP2′ outputs a low voltage, and hence transistor M3′ is turned off. Current source I_Bias′ then charges capacitor C′, and hence voltage VA′ increases over time. At the time voltage VA′ is equal to or greater than reference voltage V_reference′, operational amplifier OP2′ outputs a voltage high enough for turning on transistor M3′. Accordingly, node A′ is shorted to the electrical ground, and voltage VA′ is reduced to the ground voltage. With the positive input receiving voltage VA′ at the ground voltage, operational amplifier OP2′ outputs a low voltage to turn off transistor M3′, and current source I_Bias′ charges capacitor C′ again. With the above-described cycle repetition, a saw-tooth voltage is generated at node A′.
In the summation circuit, operational amplifier OP1′ has a positive input receiving the saw-tooth voltage VA′. The negative input of operational amplifier OP1′ is connected to resistor R0′, whose resistance is also denoted as R0′. Therefore, current I1′, which is equal to VA′/R0′, flows through resistor R0′, transistor M4′, and transistor M1′. Transistor M1′ forms a current mirror with transistor M2′, and current I2′ flowing through transistor M2′, resistor R1′, and resistor R_sense′ is proportional to current I1′. If transistors M1′ and M2′ are identical, current I1′ equals current I2′. Current I_sense′ is inserted to node B′. Accordingly, the summation voltage V_sum is:V_sum=(VA′/R0′)×(R1′+R_sense′)+I_sense′×R_sense′  [Eq. 1]
Accordingly, through the current summation of currents I2′ and current I_sense′, the summation voltage V_sum is obtained. Since current+I_sense′ represents the converted voltage, summation voltage V_sum represents the sum of the saw tooth voltage and the converted voltage.